I have an ADC212-50.
I would like to monitor the current that flow, in a tube power supply, through a test resistor (about 10 ohm) in series with the load, for 1 minute to understand what happen. Maybe is better to use a differential probe, do you think that using two channel, connected on the sides of the resistors, I will find the solution?

To start with, I assume you are talking about a Valve Amplifier PSU. There are some safety implications here! If the voltage is above 200V DC then a 10:1 probe will still output over 20V which is greater than the scope maximum range (+/-20V). If you are within range you can measure either side of the resistor and subtract the voltage difference to calculate the current but the sensitivity and resolution will be very poor. A current change of 100mA will give a voltage difference of only 1V on a 200V range. A differential probe or current probe would be much better. Failing that, a fast responding analogue meter or DVM in place of the resistor, in current mode would give some indication visually, of what is happening. What are you actually trying to capture?
PeterF.

thanks for e-mail.
My intention is to have a graphic with the shape of the current that flows. This because on tube rectification there are problems with filter capacitance when the tube start to conduct and the capacitor is not charged.
Can you suggest me the a differential probe? ( I am agree that is the best way).
Of course I have a limit of 200Vdc (with the probe at 1:10) but often I will have 450 Vdc.
Can you suggest to buy a parallel to usb adapter fort my ADc212-50?

If you need a USB to Parallel adapter for your ADC212/50 you will have to go for the Pico adapter PP196. No other manufacturers adapter will work with Pico products because of the special way in which the port is used to get maximum speed. The Pico adapter also will not work with any other manufacturers equipment for the same reason. The Pico adapter is Â£50.
Link below:-
http://www.picotech.com/usb_parallel_port.html